I use the pattern you describe, but not in Ruby. I use code to build fixtures through sql inserts. The code creates a new db whose name includes a hash of the test data (actually a hash the source files that build the fixtures).
Read-only tests only need to run the bootstrap code if their particular fixture hasn’t been created on that machine before. Same with some tests that write data but can be encapsulated in a transaction that gets rolled back at the end.
Some more complex tests need an isolated db because their changes can’t be contained in a db transaction (usually because the code under test commits a db transaction). These need to run the fixture bootstrap every time. We don’t have many of these so it’s not a big deal that they take a second or two. If we had more we would probably use separate, smaller fixtures for these.
Read-only tests only need to run the bootstrap code if their particular fixture hasn’t been created on that machine before. Same with some tests that write data but can be encapsulated in a transaction that gets rolled back at the end.
Some more complex tests need an isolated db because their changes can’t be contained in a db transaction (usually because the code under test commits a db transaction). These need to run the fixture bootstrap every time. We don’t have many of these so it’s not a big deal that they take a second or two. If we had more we would probably use separate, smaller fixtures for these.